1. Field of the Invention
The present invention relates to a data storage/access system, and a data restoration system when a system fault occurs, in a client-server system which consists of a plurality of servers and clients inter-connected.
2. Description of the Prior Art
In recent years, an enthusiastic research and development have been achieved on a video server which stores a lots of video programs such as movies, and distributes moving pictures upon a demand from a client. In a conventional video server system, when a whole system needs extension due to lack of capacity of server computers, it has been solved by replacing the old ones with a higher performance server computer, or by increasing the number of server computers so that a load of processing is distributed over a plurality of server computers. The latter way of extending the system by increasing the number of server computers is effective in terms of work load and cost. A video server as such is introduced in "A Tiger of Microsoft, United States, Video on Demand" in an extra volume of Nikkei Electronics titled "Technology which underlies Information Superhighway in the United States", pages 40, 41 published in Oct. 24, 1994 by Nikkei BP.
FIG. 31 is a block diagram showing a conventional video server system. The configuration of FIG. 31 comprises a network 1, server computers 2 which are connected to the network 1 and have a function as a video server, magnetic disk unit 3 which are connected to the server computers 2 and stores video programs, clients 5 which are connected to the network 1 and demand the server computers 2 to read out a video program. Each server computer 2 has a different plurality of set of video programs such as a movie stored in the magnetic disk units 3. A client 5 therefore reads out a video program via one of the server computers 2 which has a magnetic disk units 3 where a necessary video program is stored.
A conventional video server system where each one of a plurality of server computers stores a independent set of video program, the whole system is utilized efficiently when each demand on a video program is distributed to different server computers. However when a plurality of accesses rush into a certain video program, a work load increases on a server computer where this video program is stored, namely a work load disparity will be caused among server computers. Even if the other server computers remain idle, the whole capacity of the system has reached to the utmost level because of the overload on a capacity of a single computer. This deteriorates the efficiency of the whole system.
It is an object of the invention to provide a client-server system comprising a plurality of server computers in which a work load is distributed over the all of the plurality of server computers to improve the efficiency of the whole system, even if there is a rush of access to a certain program.
It is another object of the invention to increase the reliability of the system when a fault occurred on the system.